Guy's Behavior

Why won’t a guy just go for his girl best friend if he likes her?

My boyfriend has a girl best friend that he’s known for 5 years. They kinda hooked up when they first met but just remained friends ever since. Both have had girlfriend and boyfriend while still friends. He lived with her and her boyfriend. And she helped him get a job that kinda changed his life (used to be a big partier).

Recently I found messages on my bfs phone from his friend saying dump her and date me, I wanna kiss you and I’m prettier than her. These were 4 months ago. My boyfriend replied with no I’m with (me) and we wouldn’t be good together.
He said he doesn’t have feelings for her and I believe him by his responses that I seen. But when we broke up for two weeks, she was the first person he reached out to (the next day) saying hey let’s fuck. She has a boyfriend but was still like come over on the weekend then.

Nothing came of that, him and I got back together, they remain friends and now work together. I confronted him again about it saying I’m uncomfortable. And he laughed about it and said I’m the only one he wants, it’s so clear to him and it’s silly that I think that way because he is so certain on me and not her.
She has been there for him by putting a roof over his head and finding him a job. I feel bad asking him to stop talking to her. Should I just trust that he doesn’t like her and carry on? I feel like he would have taken the opportunity and broke up with me when she put herself out there to him if he liked her right?

Why won’t a guy just go for his girl best friend if he likes her?
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